In a baseband wireless modem design, symbol timing and initial frequency offsets are often accounted for during initial signal acquisition. However, due to differences between transmitter and receiver clock frequencies, timing and frequency errors may continue to accumulate over the duration of a data packet transfer. For longer packets, the uncorrected timing and frequency offsets accumulated throughout the packet transfer may severely degrade the receiver performance.
For example, according to the IEEE 802.15.4 specification, which is a basis for ZigBee's specification, the OQPSK physical layer (PHY) symbol rate is 25 ksymbol/s when operating in the 868 MHz band and 62.5 ksymbol/s when operating in the 780 MHz, 915 MHz, or 2450 MHz band with an accuracy of ±40 parts per million (ppm). OQPSK stands for offset quadrature phase-shift keying. ZigBee's OQPSK is a form of continuous phase frequency shift keying (CPFSK) modulation in which the instantaneous frequency is binary fk=±Δf, whereas the instantaneous frequency for CPFSK generally may be fk=±Δf, ±3Δf, . . . . Under CPFSK, each symbol has a fixed duration, and the phase from one symbol to a subsequent symbol maintains continuity.
If the transmit clock and receive clock have an accuracy within 40 ppm but in opposite direction, it is equivalent to the receiver experiencing 80 ppm offset. Hence, the maximum frequency offset possible is about 2.4 GHz*80/1M=192 kHz or just about 200 kHz. In addition, the maximum PSDU size is 128 octets, which is equivalent to 256 symbols and 8192 chips (a chip generally refers to a pulse of a direct-sequence spread spectrum (DSSS) code). Thus, in this extreme case, the total timing drift by the end of the packet is 8192 chips*80/1e6˜0.66 chips. An offset of more than half a chip can be devastating to the OQPSK demodulation performance.